As the production method of isoprene, a method comprising continuously or intermittently supplying isobutylene and/or t-butanol (hereinafter “isobutylene and/or t-butanol” is sometimes to be abbreviated as “C4”), formaldehyde and water to a reactor containing an acidic aqueous solution, and carrying out a reaction while distilling away the produced isoprene together with water and other low boiling point components to the outside of the reaction system is known (e.g., JP-A-59-70623).
In addition, a production method is known, wherein, in the production method described in JP-A-59-70623, a part of a mixture of C4, formaldehyde, water and acidic aqueous solution (hereinafter to be abbreviated as a “reaction mixture”) is taken out from the reactor, heated together with at least a part of C4, and then introduce the mixture into the reactor again (e.g., JP-A-59-190930). Note that “water” may be understood to be contained as an aqueous solution such as an acidic aqueous solution and the like, and the “reaction mixture” may contain a reaction product and by-products.
In addition, a production method of isoprene is known, which is characterized in that, in the production method described in JP-A-59-70623, the quantity of heat of water is recovered by fractionally condensing mostly water in the produced isoprene, water, unreacted starting materials and other low boiling point components, which are distilled away from the reaction system as a gas (hereinafter the “produced isoprene, water, unreacted starting materials and other low boiling point components” is to be abbreviated as a “reaction distillation gas”), the quantity of heat of an uncondensed gas is used as a heat source for vaporizing isobutylene, a heat source for recovering unreacted isobutylene by distillation from the organic layer obtained by concentration and phase separation of the reaction distillation gas, and the like (e.g., JP-A-60-4138).
Furthermore, a process for producing isoprene is known, which is characterized in that, for separation of high boiling point byproducts accumulated in a reaction mixture, a residue or a part thereof, which is obtained by evaporating unreacted starting materials and isoprene from an organic layer of a distillate during the reaction, is added to the reaction mixture or a part thereof, and the organic layer containing the high boiling point byproducts are separated from an acidic aqueous solution (e.g., JP-A-59-116236).
However, the present inventors have studied the above-mentioned conventional production methods of isoprene and newly found the following problems in these production methods.
To be specific, by simply performing the production methods described in JP-A-59-70623, JP-A-59-190930 and JP-A-59-116236, the concentration of high boiling point byproducts in the reaction mixture shows variation, which in turn unstabilizes the concentration of acid in the reaction mixture, and causes corrosion of apparatuses and degraded results of reaction to produce isoprene.
Particularly, it was found that, when a part of a reaction mixture is drawn from a reactor and heated together with at least a part of C4, the heat is not sufficiently transmitted to the reaction mixture at a concentration too high or too low of high boiling point byproducts in the reaction mixture. When the heat is not sufficiently transmitted, the temperature of the reaction mixture in the reactor becomes gradually low, and the amount of water distilled during the reaction tends to decrease. For continuous reaction, therefore, the amount of the aqueous formaldehyde solution to be supplied needs to be reduced to constantly maintain the surface of the reaction mixture at a certain level, which in turn often decreases the amount of producing isoprene per unit time.
On the other hand, when the temperature of the reaction mixture in the reactor is constantly maintained at a certain level in this situation by excessively heating the reaction mixture drawn from the reactor and at least a part of C4, some drawbacks occur including lower use efficiency of the quantity of heat necessary for the production of isoprene, easy clogging of pipe lines with high boiling point byproducts accumulated therein and the like.
In the process described in JP-A-60-4138, the quantity of heat possessed by a reaction distillation gas can be effectively recovered and reused. However, it is not satisfactory as a supplement to the quantity of heat necessary for producing isoprene, and there is a room for improvement prior to industrial practice.
It is therefore an object of the present invention to solve the above-mentioned problems and provide a process capable of producing isoprene more efficiently than conventional processes.